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(a) PMMA 팬텀을 이용한 물 흡수선량 측정

(b) 물로 채워진 기준 구형 물 팬텀을 이용한 물 흡수선량 측정

(c) 고가의 상용 Solid water 팬텀을 이용한 물 흡수선량 측정

(d) PMMA 팬텀에 radiochromic film을 장착하여 페넘브라 측정

Abstract
BACKGROUND:
To measure the absorbed dose rate to water and penumbra of a Gamma Knife® (GK) using a polymethyl metacrylate (PMMA) phantom.

METHODS:
A multi-purpose PMMA phantom was developed to measure the absorbed dose rate to water and the dose distribution of a GK. The phantom consists of a hemispherical outer phantom, one exchangeable cylindrical chamber-hosting inner phantom, and two film-hosting inner phantoms. The radius of the phantom was determined considering the electron density of the PMMA such that it corresponds to 8 g/cm2 water depth, which is the reference depth of the absorbed dose measurement of GK. The absorbed dose rate to water was measured with a PTW TN31010 chamber, and the dose distributions were measured with radiochromic films at the calibration center of a patient positioning system of a GK Perfexion. A spherical water-filled phantom with the same water equivalent depth was constructed as a reference phantom. The dose rate to water and dose distributions at the center of a circular field delimited by a 16-mm collimator were measured with the PMMA phantom at six GK Perfexion sites.

RESULTS:
The radius of the PMMA phantom was determined to be 6.93 cm, corresponding to equivalent water depth of 8 g/cm2. The absorbed dose rate to water was measured with the PMMA phantom, the spherical water-filled phantom and a commercial solid water phantom. The measured dose rate with the PMMA phantom was 1.2% and 1.8% higher than those measured with the spherical water-filled phantom and the solid water phantom, respectively. These differences can be explained by the scattered photon contribution of PMMA off incoming 60Co gamma-rays to the dose rate. The average full width half maximum and penumbra values measured with the PMMA phantom showed reasonable agreement with two calculated values, one at the center of the PMMA phantom (LGP6.93) and other at the center of a water sphere with a radius of 8 cm (LGP8.0) given by Leksell Gamma Plan using the TMR10 algorithm.

CONCLUSIONS:
A PMMA phantom constructed in this study to measure the absorbed dose rates to water and dose distributions of a GK represents an acceptable and practical alternative for GK dosimetry considering its cost-effectiveness and ease of handling.

Author information

Chung JP1, Seong YM1, Kim TY2, Choi Y2, Kim TH3, Choi HJ4, Min CH4, Benmakhlouf H5, Chun KJ6, Chung HT7.
1
Center for Ionizing Radiation, Division of Metrology for Quality of Life, Korea Research Institute of Standards and Science, 267 Gajeong-ro, Yuseong-gu, Daejon, 34311, Korea.
2
Department of Accelerator Science, Korea University Sejong Campus, 2511 Sejong-ro, Sejong, 30019, Korea.
3
Department of Nuclear Engineering, Hanyang University College of Engineering, Seoul, 04763, Korea.
4
Department of Radiation Convergence Engineering, Yonsei University, 1 Yeonsedae-gil, Heungeop-myeon, Wonju, 26493, Korea.
5
Department of Medical Radiation Physics and Nuclear Medicine, Karolinska University Hospital, SE-17176, Stockholm, Sweden.
6
Department of Accelerator Science, Korea University Sejong Campus, 2511 Sejong-ro, Sejong, 30019, Korea. chunkj@korea.ac.kr.
7
Department of Neurosurgery, Seoul National University College of Medicine, 101 Daehak-ro Jongno-gu, Seoul, 03080, Korea. htchung@snu.ac.kr.